I can't find Denoiser in 2.80
The Intel® Open Image Denoise is only available in Blender 2.81 and later as compositor node. In order to use it, enable Denoising Data in  View Layer Properties > Passes.



Switch to the compositor and add the node through Add > Filter > Denoise. Connect the Image output socket from the Render Layer to the Image input socket of the Denoise node. Use the Denoising Normal and Denoising Albedo from the Render Layer node as input for the Normal and Albedo sockets of the Denoise node.





The build-in denoiser for Cycles can be found in the View Layer tab, when Cycles is enabled as render engine.